The term "plasterboard" is used herein to refer to a composite board made from plaster (gypsum) and cardboard and used largely in buildings as a wall board or the like. When plasterboard is made, a moist plaster composite is formed into board and must be dried. For this purpose a multistage drier of the forced-air type is commonly used. A drier for broad articles such as strip material or material in the form of boards, like plasterboard, can be constructed as described in DE-OS 19 46 696 which has previously recognized the problem of uniformly drying a board over the full width of the conveyer on which the boards are displaced in a transport path through the drier.
Basically that drier comprises a conveyor means which can include or be formed by a roller conveyor defining a transport plane on which the plasterboard is displaced through a plurality of drying stages. Nozzle boxes extend transversely of the drying path, are supplied with drying air from a blower arrangement, and train the jets of drying air against the plasterboard.
The nozzle boxes themselves are wedge-shaped so that the cross section of the nozzle box decreases from the inlet side of the nozzle box toward its opposite side. The nozzle box may have inlet openings at the wider end and may be closed at its narrow end. Distributed over the side of the nozzle box which is turned in the direction of the plasterboard or other article to be dried, is a plurality of nozzles. The taper or convergence of the nozzle box enables a more or less constant pressure to be maintained at the nozzle openings over the entire length of the nozzle box.
The wedge-shaped nozzle boxes are so arranged that the spacing of the nozzle box wall juxtaposed with the article is greatest where the nozzle box is narrowest and decreases toward the inlet side of the nozzle box or, stated otherwise, the spacing of this wall from the board passing through the drier increases in the direction of the tapered end from a smallest spacing close to the inlet side.
Because the jets of air flowing out of the nozzle box have to travel different distances to reach the surface of the board to be dried, the problem of maintaining a uniform velocity of contact of the various jets with the board in this earlier system is resolved by forming the nozzle as short tube segments whose lengths are greater, the further the nozzle is removed from the inlet side of the nozzle box. The outlet ends of these tubes can be at a constant distance from the board against which the streamlets of drying air is trained. Because the spacing between the outlets of the nozzles and the board to be dried is constant over the entire width of the transport path, there is a greater uniformity of drawing action over the width than in systems in which the mouth of the nozzle orifice is at a varying distance from the material treated. This earlier drier subdivides the drying action into zones and switches the inlet side of the nozzle boxes from one side to another side of the drier from zone to zone along the transport path.
The latter feature requires that the drive for the conveyor vary from side to side along the path as well and that complicates maintenance because of the need to have access on both sides. It also complicates the conveyor drive or the operation and reliability of the drier. In spite of all of these efforts, moreover, it has been found in practice that uniform drying across the width of the path cannot always be ensured.
The nonuniformity of the drying action is noticeable particularly in systems in which the drying is carried out at multiple levels, i.e. in a multilevel drier in which conveyors are spaced one above another and corresponding arrays of nozzle boxes are provided for each of the conveyors. Such driers are highly desirable because of their relatively compact construction.
With multilevel driers, especially where the cross section of individual nozzle boxes is small by comparison to the lengths of such nozzle boxes, there can be variations in the air velocity entering the nozzle boxes and within the nozzle boxes between the ends thereof so that there will also be variations in the velocities of the air jets emerging from the nozzle boxes. Furthermore, the air jets more remote from the inlet sides of the nozzle boxes tend to deviate more sharply from the vertical than the air jets closer to the inlet side. The air flowing out of the nozzle boxes tends to cause a greater cooling of the nozzle box and hence of the air reaching the boards to be dried at the side opposite the inlet side than toward the inlet side. This also tends to reduce the drying effect at the sides of the drier at which the narrow ends of the nozzle boxes are disposed. Finally it has been found that correction and adjustment of this earlier drier construction to provide uniformity in drying action is difficult or impossible.